The field-oriented control of a permanently excited synchronous motor requires knowledge of the position of the rotor in relation to the stator. This is called an angular position within a magnetic period of the motor, or also referred to as the commutation angle. Since rotary synchronous motors frequently have many (for instance, n) pole pairs, a plurality (i.e., n) of magnetic periods corresponds to one full mechanical rotation of a rotating motor. Linear motors have periodic magnetic fields as well, so that reference is made to a commutation angle within a magnetic period here, too.
The position of a rotor in the rotating motor or the position of a rotor in the linear motor (hereinafter simply referred to as rotor) is frequently detected with the aid of a position-measuring device. This device may measure in an absolute or incremental manner and thereby provide information about the movement of the rotor. However, in the case of incremental position-measuring devices, no information is available after each activation that indicates at which angle the rotor is positioned relative to a magnetic period of the motor; and in the case of absolute measuring devices, this is the case at least after the measuring device has been attached. Yet the commutation angle is required in order to be able to apply a current vector to the motor, whose moment-generating and field-generating components are aligned with respect to the motor as required.
There are numerous methods for ascertaining the commutation angle. Apart from the many methods that have ascertained the commutation angle via a measurement of electrotechnical variables such as the inductivity or the magnetic saturation, there are also a great number of methods that are based on an energization of the motor and the ascertainment of larger or smaller deflections of the rotor.
For example, German Published Patent Application No. 102 13 375 describes impressing a movement from the outside into a synchronous motor under a field-oriented control, and specifying a setpoint current of zero for the control circuit in the process. Voltages that counteract the voltages induced in the motor by the movement are therefore adjusted by the control circuit. The phase position of these voltages makes it possible to infer the commutation angle. However, these methods require greater movements of the rotor, which are not always possible.
European Patent No. 1 495 535 describes impressing current vectors having different directions into a motor which is kept stationary by a motor brake, and recording minutest deflections of the rotor that are possible despite the motor brake on account of elasticities of the mechanics, with regard to the amount and direction with the aid of a position-measuring device. Plotting this deflection over the angle of the current vectors makes it possible to infer the commutation angle. This is attributable to the fact that the largest deflection is achievable when the rotor and current vector are situated perpendicular to each other. However, this method requires a rotor that must be kept stationary and is unsuitable for systems in motion.
Another possibility for ascertaining the commutation angle is to apply a moment-generating current of a randomly selected orientation to the synchronous motor. The rotor will move then and align itself in parallel with this direction. Its position and the commutation angle are known as a result. However, such methods are disadvantageous in cases where greater movements of the rotor are undesired or where the rotor movement is subject to very little friction, because it takes a very long time in such cases until the rotor has adjusted to the desired position.
It is for this reason that methods have also become used in which it is not the rotor that aligns itself with an applied current vector; instead, the angle of the current vector is adjusted with the aid of a control circuit, such that it aligns itself with respect to the rotor, which executes only small movements about its starting position in the process. For example, European Published Patent Application No. 0 784 378 describes impressing a current vector initially at a random direction and specifying a rotational frequency of zero in so doing. A PI controller, at whose input the deviation of the measured rotational frequency from the setpoint rotational frequency (i.e., zero) is applied, outputs a correction angle by which the direction of the applied current vector must be corrected in order to keep the rotational frequency at the setpoint value of zero. That is to say, slight movements of the rotor are detected and immediately corrected, so that the rotor does not execute any larger movements. Following a brief adjustment period, the angle output by the PI controller is constant and corresponds to the commutation angle in question. However, when testing such methods it was found that a reliable detection of the commutation angle was impossible under certain conditions. In particular in systems that are in motion and have low friction, this method was unsuccessful because the commutation offset did not adjust to a fixed value. For example, this was the case in an air-bearing-supported linear motor which is moving at an initial speed.